1. Field of the Invention
The present invention relates to a semiconductor device and more specifically to semiconductor devices in a transmitter system of a wireless door opening and closing system, each of which is composed of a timer circuit for controlling an operation time of the transmitter system, a regulator circuit for supplying power to a transmission circuit, and a memory for rolling codes.
2. Description of the Related Art
A transmitter of a wireless door opening and closing system, for example, a key system for opening and closing a door of an automobile, is battery driven and, in order to prolong the life of the battery, a built-in transmission circuit portion and a microcomputer have to operate on low current. A common method to reduce the power consumption of the transmission circuit portion and microcomputer is to lower the carrier wave frequency of radio while using a detector circuit to cause the microcomputer to operate intermittently (see JP 2000-17916 A (Pages 3 to 4, FIG. 1), for example).
Even when the carrier wave frequency of radio is lowered and a detector circuit is used to cause the microcomputer to operate intermittently as a measure to prolong battery life, the detector circuit portion consumes a large amount of current and thus presents an obstacle to exponential prolonging of battery life. In addition, lowering of the carrier wave frequency means a shorter reaching range, which makes steady transmission difficult.